Prestressed piezoelectric audio transducer

ABSTRACT

A COMBINATION PIEZOELECTRIC TRANSDUCER AND DIAPHRAGM INCLUDING A THIN CIRCULAR DISC OF POLARIZED PIEZOELECTRIC CERAMIC MATERIAL STRESSED INTO A DISH-SHAPED CONFIGURATION AND CONFINED IN THAT CONFIGURATION BY A METAL RING ATTACHED TO THE EDGE OF THE DISC. A CONDUCTIVE METAL LAYER IS FORMED ON EACH OF THE MAJOR SURFACES OF THE DISC TO FORM A PAIR OF ELECTRODES FOR THE DISC, SO THAT THE APPLICATION OF ELECTRICAL SIGNALS TO THE ELECTRODES CAUSES THE DISC TO OPERATE INAN EXPANDER MODE, THEREBY PRODUCING MOVEMENT OF THE CENTER OF THE DISC IN A DIRECTION PERPENDICULAR TO THE MAJOR SURFACES THEREOF. MOVEMENT OF THE CENTER OF THE DISC IN A PERPENDICULAR DIRECTION TO THE MAJOR SURFACES THEREOF CAUSES A CORRESPONDING ELECTRICAL SIGNAL TO BE GENERATED ACROSS THE ELECTRODES.

Q United States Patent m1 5 ,5

[72] Inventor lingo W. Schllt 2.549.872 4/1951 Willard 31018.2

- Der Plains. Ill. 2.487.962 l 1/1949 Amdt 310/96 [21] Appl. No. 860.218 2,756,353 7/1956 Samscl 31018.0 [22] Filed Sept. 23. 1969 2.386.279 10/1945 'l'ibbetta 310/85 [45] Patented Jam: 28, 1971 3,222,462 12/1965 Kannann et a1. 179/110 [73] Aaaignee Motorola, Inc.

. v Primary Exammer- D. 1-. Duggan haul. M Assirtant Examiner-B. A. Reynolds p Attorney-Mueller and Aichele [54] PRES'IRESSIZD PIEZOIZLECIRIC AUDIO h TRANSDUCER lat-s, 8

n c nn'h' ABSTRACT: A combination piezoelectric transducer and 1521 cl 310/337- diaphragm including a thin circular disc of polarized 3 10/91, 310196 iezoelectric ceramic material stressed into a dish-shaped 1 1 Ill-cl 7 configuration and confined in that configuration by a metal O M 31018.2. in mchgd w [hg edge of 1b: digg A conduct; mega] layer 8.8, 8.3, 9.1, 9.4, 8.5, 9.6. 8.7; 3 0/" is formed on each of the major surfaces of the disc to form a v pair of electrodes for the disc. so that the application of elecdmcm trieal signals to the electrodes causes the disc to operate in an UNITED STATES 515m 5 expander mode, thereby producing movement of the center of 2.836.738 5/1958 Crownover 31018.5 the disc in a direction perpendicular to the major surfaces 3.365.593 1/1968 Rool'et a1. 31018.7 thereof. Movement of the center of the disc in a perpendicular 3,271,596 9/1966 Brinkerhoi'f. 3 l0/8.7 direction to the major surfaceathereof causes a corresponding 3.380.019 4/1968 Sims 340/10 electrical signal to begenerated across the electrodes.

- IO; 1 l I WEIGHT PATENTEHuuuz 8 I97! AUDIO OUTPUT FREQUENCY (FIXED E-|NPUT) INVENTOR HUGO W. SCHAFFT avg-Maegan, QRQMQ.

ATTORNEYS.

PRISTRESSED PIEZOELECI'IIIC AUDIO TRANSDUCER BACKGROUND OF THE INVENTION enough to provide the proper impedance matching with air loading.

suitsrARYoF rm: mvr-zrmon Accordingly it'is an object of this invention to provide an improved piezoelectric transducer device. I

It is another object of this invention to provide a piezoelectric transducer which also operates directly as a diaphragm coupled to air.

It is an additional object of this invention to use a thin preatressed slab of piezoelectric material as a combination electrical to mechanical transducer and diaphragm operable at audio frequencies and coupled to air.

In accordance with a preferred embodiment of this invention a single thin slab of piezoelectric material. capable of operating in an expander mode. is placed under stress and polarized. The slab is clamped in the stressed configuration by attaching it in an opening in a frame corresponding to the shape of the slab. and electrodes are placed on the opposite major surfaces of the slab for the application of electrical impulses to and the obtaining of electrical signals from the slab. The frame is relatively rigid in the plane of the opening but is capable of torsional movement about an axis in the fram parallel to the plane of the opening.

' BRIEF DESCRIPTION OF THE DRAWING FIG. I is a drawing illustrating the method of making a piezoelectric transducer/diaphragm in accordance with a preferred embodiment of this invention;

FIG. 2 is a perspective view of the completed diaphragm made in accordance with FIG. I;

FIGS. 3 and 4 are partially cutaway side views illustrating operation of the piezoelectric transducer/diaphragm made in accordance with the method illustrated in FIG. I;

FIG. 5 is a top view of the diaphragm shown in FIGS. 2. 3 and 4 used to explain the operation of the diaphragm;

FIG. 6 is a partially cutaway side view of a device useful in explaining the operation of the diaphragm;

FIG. 7 shows typical response curves used to determine the proper parameters of the transducer/diaphragm, and

FIG. 8 is a detailed partial sectional view of another embodiment of the invention.

DETAILED DESCRIPTION Referring now to FIG. I. there is illustrated, in partially cutawny side view. a method for making a combination piezoelectric transducer/diaphragm. A circular piezoelectric disc 10 made of a suitable polycrystalline material. such as barium titanate. is coated with conductive layers on both of the major surfaces to form electrodes II and I2. which may be applied to the disc or slab I0 in any suitable manner. such as by plating. sputtering or painting. The diameter of the disc 10 is substantially greater than the thickness of the disc, although in FIG. I. the relative dimensions have been exaggerated in order to show more clearly the elements of the transducer.

The disc III with the electrode coatings II and 12 then is placed in a groove in a ring I3 preferably made out of material having a high Young's modulus of elasticity. The groove or cutaway portion of the ring is defined by a vertical surface I4 and a lower surface IS. with the disc l0 resting on the lower surface I5 and being distorted by the application of a weight indicated by the arrow applied approximately at the center of the disc. This causes the disc to assume the dish-shaped configuration shown in FIG. I and to rest on the lower surfaces 15. with the edge of the disc It) being within the confines of the diameter across the ring I3 between the surfaces I4.

When the disc I0 is in the position shown in FIG. I. with the weight applied to its center. a DC polarizing potentialmay be applied to the electrodes II and I2. causing a unidirectional (DC) electrical field to be established between the electrodes II and I2 and. therefore. through the ceramic disc 10. This DC field should be sufficiently high to cause a polarization of the ceramic disc 10 to be effected. If the deforming pressure and the electrical field is applied at normal room temperature. it generally is necessary to use a relatively high potential and to maintain the deformation and potential for several hours. Alternatively. heat may be applied to the disc to raise its temperature above the Curie point before the DC potential is applied. and then the heat is removed, permitting the disc to cool through the Curie point. while the pressure and electrical field are maintained. If polarization of the ceramic disc I0 is accomplished by this latter method. a lower polarizing electrical field and a shorter time length generally can be employed.

While the disc I0 is distorted by the weight and has the electrical field applied to it, the edges of the disc III are cemented by a suitable cement. such as an epoxy cement. into the groove formed by the surfaces I4 and IS on the metallic ring I3; and the epoxy cement is allowed to set. After the epoxy has set. the weight and DC polarizing field are removed. The epoxy forms a rigid bond between the edge of the disc I0 and the ring. so that the disc is held in the distorted dish-shaped configuration which it attained under the action of the weight applied to it. The resultant transducer/diaphragm is shown in FIG. 2.

The piezoelectric diaphragm assembly shown in FIGS. I and 2 may be coupled directly to air; because the single layer I polycrystalline ceramic piezoelectric disc I0 may be made thin enough. thereby having a low enough impedance. to provide a sufficient impedance match to air that audio frequency vibrations may be directly coupled to and from the disc using amplifiers of the type normally employed in small radios. telephones and the like. Before entering into a discussion of the operation of the transducer/diaphragm assembly shown in FIGS. I and 2. refer to FIG. 6 which illustrates an idealized configuration of the-assembly useful in explaining the principles of operation of the device. The device shown in FIG. 6 is substantially the sameas the device shown in FIGS. I and 2. but employs a different ring configuration in the form of a ring 23. having a cross-sectional configuration such that the inner surface of the ring forms a circular ltnife edge which is clearly shown in the crosssectional view of FIG. 6. The piezoelectric slab or disc I0 then is prestressed into a dished configuration and is wedged into the opening in the ring 23. with the knife edge engaging .the edge of the disc 10 substantially at the midpoint of the thickness of the disc all around its circumference. As a remit. the disc I0 is constrained against movement along any diameter because of the rigid nature of the material of the ringifi.

The application of an alternating current from a current generator 20 causes an alternating field to be applied across the disc III by means of the electrodes II and I2. In the presence of an alternating field. the disc alternately expands and contracts along its diameters. Since the disc I0. however. is constrained against movement in the direction of its diameters. this expansion and contraction is translated into a movement normal or perpendicular to the general plane of tire disc 10. as indicated by the vertical double ended arrow in FIG. i. As the disc I0 expands. the amount of the dishing of the disc increases. causing a downward movement of the center of the disc I0 as seen in FIG. 6. Similarly. as the disc contracts along its dian. r-ters. the amount of dishing decreases. causing an upward movement of the center of the disc 10 as viewed in FIG. 6.

Because the amount of the "dishing" of the disc varies in accordsnce with the signals applied to the electrodes II and I) there is relatively friction-free rotation or a hinging of the disc at the knife edge contact point along the edge of the disc I0. The embodiment shown in FIG. 6. however. is diflicult to attain as a practical matter due to the difficulty of placing and holding the disc 10 in place on the knife edge of the ring 23. and also due to the extremely large pressures exerted on the disc I by the line contact of the knife edge. As a consequence. the compromise configuration shown in the FIGS. 1'. 2. 3. and 4 of the drawing proves tube the most practical from an actual operating standpoint.

In order most closely to duplicate the operation of the embodiment shown in 'FIG. 6. it is desirable that the ring l3 used in the assembly previously described in conjunction with FIGS. I- and 2 be made of a material having a high Young's modulus. providing a high torsional elasticity. but also providing a maximum rigidity to the expansion and contraction of the disc I0 along its diameter. Materials out of which the ring 13 may be fabricated and which exhibit these desired characteristics are tungsten. tungsten carbide. alumina ceramics. molybdenum and the like. The range of Youngsmodulus of these materials extends from approximately 320- I000 xl00.000 p.s.i. (pounds per square inch) at room temperature. The cross-sectional diameter of the ring preferably should be small to provide the desired torsional characteristics and should be large to provide maximum resistance to expansion and contraction of the disc 10 in the plane of the ring 13. It has been found that by making the cross-sectional diameter of the ring 13 approximately twice the thickness of the disc I0 (for a disc 4.7 mils thick and Z-inches diameter). a good compromise can be reached.

Referring now to FIGS. 3. 4 and 5. the operation of the device shown in FIGS. I and 2 illustrated. When an alternating current signal of a first polarity from the source 20 is applied to the electrodes II and 12. the piezoelectric disc I 0 expands as indicated by the arrows placed underneath the disc I0 in FIG. 3. As illustrated in FIG. 5. the disc I0 expands along every diameter thereof. Since the ring 13 is chosen to have sufficient rigidity to prevent expansion of the disc I0 in the plane of the ring 13., the expansion of the disc I0 is translated into a downward motion at indicated by the arrow above the disc 10 in FIG. 3. causing the disc to be dished further or deeper than the configuration it attains when no signals are present from the source 20. The downward movement or dishing of the disc I0 causes the edge of the disc to pivot inwardly at the upper surface of the disc. and the modulus of elasticity of the material forming the ring I3 permits a tersional rotation of the ring I3 about a circular axis passing through the center of the circle formed by each cross section of the ring all around the ring 13. This rotation is indicated by the arrows to the left and right of the ring I3 in FIG. 3. The torsional rotation of the ring 13 functions to approximate the ideal hinge condition provided by the knife edge configuration of the apparatus shown in FIG. 6.

0n the opposite half-cycles of the alternating signal obtained from the source 20. the material of the disc 10 contracts slong every diameter thereof as seen in FIG. and as illustrated by the arrows beneath the disc I0 in FIG. 4. causing the center of the disc to move upwardly. as indicated by the arrow above the disc 10 in FIG. 4. which in turn causes the edge of the disc I0 to rotate in the opposite direction. The ring I0 to the ring t3 must provide a rigid bond.

I3 then undergoes torsional rotation in the opposite direction I about the axis thereof. as indicated by the arrows on the left and right sides of the ring IS in FIG. 4.

This expansion and contraction of the disc I0 is repeated for every cycle of the signal-obtained from the sc-urcelo. causing the bow or dishing of the disc 10 to alternately increase and decrease in the direction of the vertical arrows shown in FIGS. 3 and 4. This movement of the dis: drives the If the material of the ring I3 did not provide the torsional elasticity described but if the ring I3 were rigid in response to torsional forces. the hinging action which has been described would not occur. and distorted bending of the disc I0 at its edge would result in high mechanical losses. substantially reducing the efliciency of the device. The diameter of the disc I0 is chosen to be relatively large compared to' the thickness.

so that a high transformation ratio is attained. since a relatively small radial movement of the disc 10 provides a high axial movement at the center of the disc.

7 Referring now to FIG. 7. there is shown a chart of response curves which are used in order to determine the proper impedance matching of the transducer/diaphragm assembly shown to I118 air loading the disc I0. One means by which the proper impedance matching may be determined is to provide a fixed energy input to a number of different samples of assemblies. varying one parameter of the assemblies at a time and measuring the audio output frequency response for each of the different assemblies being tested. For example. assume that a given diameter of the disc I0 and a given material and configuration for the ring 13 are provided for a number of different samples. each having a different thickness of the disc I0. The audio frequency inputs then are applied in a similar manner to each of the samples. with the audio output response curves for each of the samples being measured.

If the disc I0 is too thick, a response curve such as the curve A is obtainedtending to provide a high audio response or a peak at a particular frequency and to provide highly attenuated audio responses over the remainder of the frequency range in which it is desired to utilize the transducer/diaphragm assembly. On the other hand. if the disc to is too thin. thereby causing too great a compliance of the disc 10. a frequency response curve such as curve C is observed. providing a relatively flat audio response characteristic. but too highly attenuated to be useful. When approximate matching of the impedance of the disc I0 to the air loading the transducer is obtained. a curve such as curve B is observed with a relatively techniques may be utilized to attain this impedance matching.

but the above technique has been found successful in producing properly matched assemblies.

Since the diameter of the disc 10 is relatively large compared to the thickness of the disc I0. the piezoelectric ceramic material may be subject to fracturing under some conditions of operation. In order to substantially eliminate or to minimize the tendencies for the thin piezoelectric material in the disc I0 to fracture. a thin layer of plastic material. such as Mylar. is cemented to one or both surfaces of the disc I0. FIG. 8 illustratcs a partial cross section of the disc I0 with the electrodes II and I2 deposited thereon and further including a layer of Mylar over the electrode I2. The Mylar layer 16 could be placed directly on the disc I0 with the electrode I2 placed over the Mylar layer if so desired. In the operation of discs having such a Mylar layer. it has been found that substantially no fractures occur. thereby resulting in greatly improved mechanical characteristics of the assembly. In a typical assembly. the ceramic material of the disc I0 may be of the order of 4.7 mils in thickness, with the Mylar layer being 0.25 milsthiclt. i

' In the operation of the transducer/diaphragm described above. a relatively wide band of response over the audio frequency range has been obta ned. In order further to improve the frequency range of the transducer/diaphragm. it may be desirable to replace one or both of the electrodes II and 12 with a resistive conductive coating. In order to improve the low frequency response of the device. it also may be desirable to attach a lightweight conical diaphragm such as a paper cone. to the disc) substantially at the center point thereof. although in most applications such an additional paper diaphragm shou=d not be necessary.

In an actual design of the transducer/diaphragm shown in FIGS. 1. 2. 3. 4. and 5, the following materials and dimensions were used:

The above values are given, however. only by way of illustration and are not to be construed as limitingthe scope of the present invention in any manner.

I claim:

I. A piezoelectric transducer including in combination:

frame means defining an opening having predetermined dimensions thereacross. the frame means having an axis therein in a plane substantially parallel to the plane of the opening and being relatively rigid in the plane of the opening to maintain said predetermined dimensions. and the frame means being capable of torsional movement about the axis;

a thin slab of piezoelectric ceramic material operable in an expander mode and of a shape generally conforming to the open ng in the frame means. the ceramic slab being bent to an extent less than its elastic limit along at least one major dimensi n thereof, with the edges thereof fitting in the openin in the frame means;

a conductive layer on at least a portion of each of the opposing major surfaces of the slab of piezoelectric material to form a pair of opposing electrodes on the slab; and

means for attaching the slab to the frame means in the openrng.

1. The combination according to claim I wherein the frame means is made of a material having a Young's modulus o elasticity greater than 310x? p.s.i.

3. The combination aecording to claim 2 wherein the frame is a ring having a groove therein and wherein the piezoelectric tungsten carbide. tungsten. alumina ceramics. and molybdenum.

l0 6. The combination according to claim 1 further including a thin layer of plastic material bonded to and substantially covering at least one of the major opposing surfaces of the slab, the thickness of the plastic material being substantially less than the thickness of the slab.

7. The combination according to claim 6 wherein the layer of plastic material is a layer of Mylar.

8. A piezoelectric transducer/diaphragm operating at audio frequencies and coupling directly to air including in cornbina tion:

a prestresaed dish-shaped piezoelectric circular disc operable in an expander mode and the diameter of which is substantially greater than the thickness thereof;

a rigid relatively nonexpansible ring. having an axis having an inner diameter sufficient to maintain the piezoelectric disc in the dish-shaped confi with a predetermined bow therein. the ring g capable of torsional movement about the axis thereof;

means for rigidly attaching the-edge of the piezoelectric disc to the inner surface of the ring; and

electrodes on at least a portion of the opposing major sur faces of the ceramic disc for applying electrical signals to and obtaining electrical signals from the disc. the die forming the sole diaphragm for the transducer.

9. The combination according to claim 8 wherein the ring has a groove therein and wherein the edge of the piezoelectric disc is placed in the groove.

10. The combination according to claim 9 wherein the ring is made of a material having a Young's modulus of elasticity greater than 3 lOX l0 p.s.i.

l l. The combination according to claim 10 wherein the material of the ring is selected from the group consisting of: tungsten. alumina ceramics, molybdenum, tungsten carbide.

12. The combination according to claim 8 further including a Mylar layer attached to and substantially covering at least one of the major surfaces of the ceramic disc, the thickness of the Mylar layer being substantially less than the thicknes of the disc. 

